Free cutting alloys



, Patented Jan. 1936 PATENT OFFICE mar: cn'r'rmc armors Louis w. Kemp!and Walter apean, Cleveland,

" Ohio, assignors America, Ifennsylvania No Drawings.

4 Claims.

This invention relates to aluminum base alloys and it is particularlyconcerned with those alloys containing silicon as a major alloy-component.

This application is a continuation in part of our copending applicationSerial No. 689,880, filed September 18,. 1933. Alloys disclosed but not.

claimed herein are claimed in our above-mentioned' copending applicationSerial No. 689,880

and in our copending applications Serial Nos.

19,607; 19,608; 19,610; 19,611; 19,612; 19,613; 19,614; 19,615, and19,616, filed May 3, 1935.

' Aluminum base alloys 3 to per cent of silicon have been widely used incast, extruded and wrought forms because of such desirable castingcharacteristics as fluidity and low solidification shrinkage of themolten metal, combined with a good workability of the solid metal if awrought article is to be made.

- The alloys also possess a low-coefficient of thermal expansion andsatisfactory physical proper.- ties, particularly the tensile strengthand elongation.- In spite of these advantageous properties, the field ofutility of these alloys has been restricted because of their inherentlypoor machining quality and consequent increased cost offinishing,articles of these alloys by machining operations such asboring, drilling, shaping, planing, or lathe-cutting. Difliculty inmachining is made evident through an excessive cutting tool wear whichnecessitates a frequent resharpening. Poor machinability is alsomanifested in a rough irregular machined surf-ace and the tendency toform long chips that may foul the cutting tool or operating parts of themachine. Aluminumsilicon alloys are furthermore quite likely to havehard particles of elementary silicon distributed throughout the alloymatrix which seriously interfere with'smoothcutting of the art'cle beingmachined. These diillculties are most readily "apparent in machiningoperations conducted on a large scale, such as the manufacture ofpistons for internal combustion engines, where it has been founddesirable to use an expensive cutting tool material to insure long toollife and a satisfactory machined surface.

It is accordingly one of the objects of our invention to makealuminum-silicon alloys amenable to machining operations on alarge-scale without encountering the disadvantages hereinbefore referredto. A specific purpose is to diminish the irregularity in cut caused bythe hard silicon particles and .to produce a smooth pleasing 'sur-' faceon the machined article. A further. object is 55 to accomplish theforegoing ends without disadcontaining from about,

to Aluminum Company of Pittsburgh, Pa

a corporation of Application May 3, .1935, erial No. 19,609

vantageously affecting the physical properties of the alloy.

Our invention is pr dicated' upon the discovery that the addition oflead and/or thallium to aluminum-silicon alloys containing from about 53 to 15 per cent silicon greatly improves the machining quality of thistype of aluminum alloy. Theimprovementis especially marked in thosealloys containing from about 10 to 15 per cent of silicon which haveheretofore been particul0 larly. difficult to machine. We have foundthat the addition of from about 0.05 to 10 per cent of lead and/orthallium to aluminum-silicon alloys imparts a machinability to the alloywhich renders it adaptable to all ordinary machining re- 15 quirements.Forthe purpose of our invention, lead and thallium are substantiallyequivalent and are considered to constitute a class of elements withrespect to their effect upon the machining quality of aluminum-siliconalloys.

The improvement in machining characteristics referred to above may bebrought about by the addition of between about 0.05 and 10 per cent oflead or thallium to aluminum base alloys containing from about 3 to 15per cent silicon, but for many applications we prefer to use betweenabout 1 and 5 per cent of either lead or thallium. The eflect onmachinability of these added elements is manifested by the smooth flowof chips from the article being machined, the short breakable chips thatare produced, the absence of gallingor unevenness of cut, resulting inan irregular surface of dull appearance, and the increase in cuttingspeed which is possible under such favorable conditions. As an exampleof an alloy possessing these properties the following composition may becited: 12 per cent silicon, 4 per cent -lead, balance substantially allaluminum. Lead and thallium may not only be success-. fully used inbinary aluminum-silicon alloys, but also in alloys containing copper,magnesium, manganese and the like in-addition to the silicon. 'Theaction of lead and thallium 'in improving the machinability of suchalloys appears to be substantially independent of the other alloyingingredients added to alter the character of the basic aluminum-siliconalloy; As an illustration of the benefit derived from the use of lead or7 thallium in alloys of this nature, the case of an alloy used in themanufacture of pistons for internal combustion engines may be cited. Analloy containing about 12.5 per cent of silicon, 1

per cent of magnesium, 0.8 per cent of nickel, and

0.8 per cent of copper, balance aluminum, had

greatly improved machining quality after" the addition of about 3 percent of lead as compared to the same alloy containing no lead. Anotherexample of the beneficial efiect of lead on the machiningcharacteristics of aluminum-silicon base alloys is found in an alloycontaining about 5 per cent silicon, 0.7 per cent copper, and 0.3 percent magnesium, the balance being aluminum. This alloy in cast form maybe used for making cylinder heads for certain types of internalcombustion engines.' It is desirable that such a casting be readilymachinable without diminishing the strength at ordinary or elevatedtemperatures. We have found that the addition of about 2.25 per cent oflead produces a marked improvement in machining quality withoutimpairing any of the other desired properties of the alloy.

Lead and thallium may not only be used separately in alloys of the typeherein disclosed, but

they may be eifectively employed in combination in certain applications.The two elements frequently co-act in an aluminum-silicon alloy toproduce a degree of machinability not attained by an equivalent amountof either element alone. When so used in combination the total amountshould not exceed 10 per cent and preferably less than 5 per cent isrecommended. The relative proportions of the two elements may be variedbut approximately equal amounts of them generally give satisfactoryresults.

For certain applications it has been found desirable to add otherelements than lead and thallium which also serve ,to improve themachining quality of the alloy. The elements cadmium and bismuth belongto this class of substances and may advantageously be combined with leadand/or thallium. When so used, the metals cadmium and bismuth should beadded in quantities of between about 1.5 and 6 per cent separately orincombination, the total amount in no case exceeding about 6 percent. Analloy of this type which has been found to have a satisfactory machiningquality is one containing about 12 per cent of silicon, 2 per cent oflead, and 2 per cent of cadmium, the balance being substantially allaluminum.

- We have also materially improved the machining quality ofaluminum-silicon alloys by using bismuth and cadmium in conjunction withlead. The three elements lead, bismuth and cadmium unite to produce abetter machining alloy than is obtainable by the use of lead. alone.When cadmium is added, the amount of bismuth may be reduced, thusdecreasing the cost of the alloy since cadmium is cheaper than bismuth.From about 1.5 per cent to 6.0 per cent each of bismuth and cadmium maybe advantageously employed in combination with the lead inaluminumsilicon alloys. The total amount of lead, bismuth and cadmiumshould not in any case exceed 10 per cent, and for most purposes a totalof less than 7 per cent gives very satisfactory results.

The tensile properties of the alloys herein described are not materiallyaffected by the addition of lead and/or thallium in amounts of less than5 per cent but there is a decline in strength if larger quantities areadded. However, the machinability of the alloy is somewhat better insuch cases which compensates in part for any loss in strength. Thetensile properties are likewise affected if both elements are presentand the total amount of lead and thallium exceeds about 5 per cent. 10

The lead and thallium may be most conveniently added to the moltenaluminum-silicon alloy in solid metallic form. Other methods of makingalloy additions practiced by those skilled in the art yield equallysatisfactory results. If the 15 amount of lead and thallium to be addedexceeds about 1.5 per cent, the melt should be heated somewhat aboveordinary temperatures and vigorously stirred to assure a uniformmixture. The method of adding lead and thallium to aluminum 20 and itsalloys here referred to is more fully described in copendingapplication, Serial No. 689,-

-885 now issued as U. S. Patent No. 1,959,029.

The term aluminum used herein and in the appended claims embraces theusual impurities 25 found in aluminum ingot of commercial grade, orpicked up in the course of the usual handling operations incident toordinary melting practice.

The alloys herein disclosed may be subjected to the usual thermaltreatments familiar to those 3 skilled in the art for the purpose ofimproving 'or altering their physical characteristics.

We claim:

1. An aluminum base alloy containing from about 3 to 15 per cent ofsilicon, from about 0.05 35 to 10 per cent of lead, at least about 1.5per cent of cadmium and at least about 1.5 per cent of bismuth, thetotal amount of cadmium and bismuth being between 3 and 10 per cent, thebalance being aluminum.

2. An aluminum base alloy containing from about 3 to 15 per cent ofsilicon, from about 1 to 5 per cent of lead, at least about 1.5 per centof cadmium and at least about 1.5 per cent of bismuth, the total amountof cadmium and bismuth being between 3 and 10 per cent, the balancebeing aluminum.

3. An aluminum base alloy containing from about 10 to 15 per cent ofsilicon, from about 0.05 to 10 per cent of lead, at least about 1.5 percent 50 of cadmium, and at least about 1.5 per cent bismuth, the totalamount of cadmium and bismuth being between 3 and 10 per cent, thebalance being aluminum.

4. An aluminum base alloy containing from 55 about 10 to 15 per cent ofsilicon, from about 1 to 5 per cent of lead, at least about 1.5 per centof cadmium and at least about 1.5 per cent of bismuth, the total amountof cadmium and bismuth being between 3 and 10 per cent, the balancebeing aluminum.

LOUIS W. KEMPF. WALTER A. DEAN.

